The ageing of organic polymers or vulcanizates thereof can lead to a change of various properties like e.g. an increased hardness or brittleness of the polymers. In the alternative a softening, a loss of the elastomeric properties or of the mechanical strength is recognized. Cracking, surface changes or other changes e.g. of the electric properties may also be observed. Undesirable odours and discolouration phenomena are also often observed.
The aforementioned property changes and phenomena are due to different ageing processes as described e.g. in Handbuch für die Gummi-Industrie, 2. völlig neu bearbeitete Ausgabe, 1991, Seite 423 ff, Bayer AG, Geschäftsbereich Kautschuk, Anwendungstechnik.
In order to prevent or reduce said ageing processes it is known in the art to add anti ageing compounds which can be typically grouped into three different categories:                (i) mono- or oligofunctional secondary aromatic amines        (ii) mono- or oligofunctional substituted phenols        (iii) heterocyclic mercapto group containing compounds.        
The effect of said anti-ageing compounds typically slows down when the polymer or vulcanizate to be protected is exposed to higher temperatures, in particular for longer time periods. Further on it is desirable that the anti ageing compounds to be used do not have a colouring effect per se, but just to the contrary provide a good colour stability to the rubber or vulcanizate thereof and may be used in combination with peroxide or sulfur based vulcanizing agents. Additionally some of the known anti ageing compounds are toxicologically risky, which means that the rubber/vulcanizate stability is only achieved by using harmful substances. Besides the diphenyl amines this applies e.g. to phenolic antioxidants like Vulkanox® BKF which is categorized as H361f, i.e. suspected of damaging fertility.
As there is an increasing need for a high ageing stability of rubbers and vulcanizates thereof with regard to storage and colour stability, in particular under exposure to high temperatures, it is a continuing task to provide new and improved concepts for preventing and reducing the ageing processes in rubbers and vulcanizates. This aim also encompasses reducing the amount of anti ageing compounds to the extent possible without lowering the stabilizing effect.
In particular rubbers with unsaturated C═C double bonds in the polymer chain like nitrile rubbers or styrene-butadiene rubbers are subject to ageing phenomena.
Nitrile rubbers and processes for producing such nitrile rubbers are known, see, for example, W. Hofmann, “Nitrilkautschuk”, Berliner Union Stuttgart 1965, pages 51-54, however, no indication is provided how to further improve the storage stability of nitrile rubbers.
For the purposes of the present invention, nitrile rubbers, also referred to as “NBR” for short, are rubbers which are copolymers or terpolymers comprising repeating units of at least one α,β-unsaturated nitrile, at least one conjugated diene and optionally one or more further copolymerizable monomers. Partially or fully hydrogenated nitrile rubbers, also referred to as “HNBR” for short, are corresponding co- or terpolymers in which the C═C double bonds of the copolymerized diene repeating units are partially or fully hydrogenated.
For many years, NBR and HNBR have occupied an established position in the specialty elastomers sector. They possess an excellent profile of properties in the form of excellent oil resistance, good heat stability, excellent resistance to ozone and chemicals, wherein the heat stability being even more pronounced in the case of HNBR than in the case of NBR. NBR and HNBR also have very good mechanical and performance properties. For this reason, they are widely used in a wide variety of different fields of use, and are used, for example, for production of gaskets, hoses, belts and damping elements in the automotive sector, and also for stators, well seals and valve seals in the oil production sector, and also for numerous parts in the electrical industry, mechanical engineering and shipbuilding. A multitude of different types are commercially available, and these feature, according to the application sector, different monomers, molecular weights, polydispersities and mechanical and physical properties. Besides the standard types, there is increasing demand particularly for specialty types featuring contents of specific termonomers or particular functionalizations.
The storage stability as well as the colour stability of rubbers like nitrile rubbers, styrene butadiene rubbers (“SBR”) or other types are frequently problematic. For the present purposes, storage-stable means that the Mooney viscosity as important specification criterion for many rubbers changes as little as possible during prolonged storage times, in particular at high temperatures. Furtheron for the present purposes, colour stable means that the rubber shows values ΔE as small as possible if determined according to CIEDE 2000 after storage at high temperatures.
JP 75,105,746 describes heat-resistant nitrile rubbers which are obtained by carrying out the coagulation of the latex by means of a mixture of tin dichloride and calcium chloride. The use of tin salts, however, is nowadays problematical for ecological reasons, especially since these tin salts are found in the nitrile rubber even after comprehensive subsequent washing of the nitrile rubber. The removal of the tin salts from the washing water is also associated with a high and therefore likewise undesirable outlay for purification.
According to Angew. Makromol. Chem. 1986, 145-146, 161-179, one measure for improving the storage stability of nitrile rubber is selective hydrogenation of the double bonds originating from the butadiene while at the same time retaining the triple bonds of the nitrile groups. The property changes achieved by the hydrogenation are desirable for many applications, but not for all. In addition, the hydrogenation is complicated and requires a series of additional process steps. As the glass transition temperatures are usually made compared to the unhydrogenated starting material by the hydrogenation such hydrogenation is not a suitable solution to the problem for all applications.
NBR is produced by emulsion polymerization, which firstly gives a NBR latex. The NBR solid is isolated from this latex by coagulation. Salts and acids are used for coagulation. In the coagulation of latices by means of metal salts, it is known that significantly larger amounts of electrolyte are required in the case of monovalent metal ions, e.g. in the form of sodium chloride, than in the case of polyvalent metal ions, e.g. in the form of calcium chloride, magnesium chloride or aluminium sulphate (Kolloid-Z. 154, 154 (1957)). It is also known from Houben-Weyl (1961), Methoden der Org. Chemie, Makromolekulare Stoffe 1, p. 484 that the use of polyvalent metal ions leads to “at least some inclusion of the emulsifier in the product”. According to these references the electrolytes used have to be very carefully washed out again, but the finished product should also be free of the catalysts and emulsifiers of the process batch. Even small amounts of residual electrolytes may result in a deterioration of various properties.
According to the teaching of DE-A 30 43 688, it is possible to reduce the amount of electrolytes necessary for the latex coagulation by using either plant-based protein-like materials or polysaccharides such as starch and if appropriate water-soluble polyamine compounds as auxiliaries in addition to the inorganic coagulate in the electrolyte coagulation of latices. As inorganic coagulates, preference is given to alkali metal or alkaline earth metal salts. The specific additives make it possible to achieve a reduction in the amounts of salts used for quantitative coagulation of the latex. DE-A 3 043 688 gives no information how to improve storage and colour stability as a result of the production and/or work-up of the nitrile rubber.
The object of EP-A-1 369 436 is to provide nitrile rubbers having a high purity. The process of EP-A-1 369 436 starts out from typical nitrile rubbers. Nothing is said about the polymerization process except that an emulsion polymerization is carried out in the presence of salts of fatty acids and/or resin acids as emulsifiers. This is followed by coagulation of the latex by means of acids, optionally with addition of precipitants. As acids, it is possible to use all mineral and organic acids which allow the desired pH values to be set. In addition, it is possible to use additional precipitates, with mention being made of alkali metal salts of inorganic acids, e.g. sodium chloride and sodium sulphate, for this purpose. The fatty acids and resin acids formed as a result of the action of acid are subsequently washed out by means of aqueous alkali metal hydroxide solutions and the polymer is finally subjected to shear until a residual moisture content of less than 20% is obtained. As a result of this shearing action, the water or the residual moisture including the ion contents and other foreign substances present therein are removed. The Ca contents of the products disclosed in Examples 1 and 2 are only 4 and 2 ppm, respectively. EP-A-1 369 436 gives no information on the production of nitrile rubbers which display increased storage and colour stability.
EP-A-0 692 496, EP-A-0 779 301 and EP-A-0 779 300 all describe specific nitrile rubbers. The nitrile rubbers contain 10-60% by weight of unsaturated nitrile and have a Mooney viscosity in the range 15-150 or, according to EP-A-0 692 496, in the range 15-65 and all contain at least 0.03 mol of C12-C16-alkylthio group per 100 mol of monomer units, with this alkylthio group having at least three tertiary carbon atoms and a sulphur atom which is bound directly to at least one of the tertiary carbon atoms. The nitrile rubbers are in each case produced in the presence of a C12-C16-alkyl thiol having a corresponding structure as molecular weight regulator which functions as “chain transfer agent” and is thus incorporated as end group into the polymer chains.
With regard to the coagulation of the latex, EP-A-0 692 496, EP-A-0 779 301 and EP-A-0 779 300 disclose that any coagulants or single alkylated phenolic anti ageing compounds which are not further specified can be used. The focus is on nitrile rubbers which are essentially halogen-free and are disclosed to have a halogen content of not more than 3 ppm and which are claimed to have an advantageous property profile, good processability of the rubber mixtures and make low fouling of the mould possible during processing. The vulcanizates obtained are said to have a good combination of low-temperature resistance and oil resistance and possess good mechanical properties. Nothing is said about the property of storage and colour stability of the nitrile rubbers and vulcanizates thereof.
EP-A-0 488 550 describes stabilizer compositions comprising 1) sulfide having one or more sulfide groups —CH2—S—CH2—R, wherein R is C1-C20 alkyl, alkyl alkanoate or 2,4-bis(n-octylthiol)-6-4′-hydroxy-3′,5′-di-tert.butylanilino) 1,3,5-triazin and at least two hindered phenols (2) and (3) one of which (3) is less sterically hindered than the other (2). Such compositions can be incorporated into polymers to make polymeric additives. These polymeric additives can be employed in polymer matrices to provide polymeric products having improved physical and mechanical properties. They are used in high concentrations of 1-4% by weight of the polymer and the focus lies on stabilizing acrylate-based rubbers. The use thereof shows some synergistic effect, however, there is neither any showing of an improved stability of the rubber's molecular weight nor any disclosure or teaching how to reduce the amount of the stabilizing system.
In U.S. Pat. No. 5,116,534 a combination of three different stabilizers is disclosed which are claimed to stabilize a broad variety of polymers. The combination consists of (i) a phenolic antioxidant, (ii) a thiodipropionic acid ester and (iii) a phosphite. With regard to the phosphite (iii) to be used alkyl substituted phenyl phosphites like TNPP (tri-nonylphenyl phosphite) are emphasized. Nowadays said phosphites, however, are considered harmful in view of their toxic by-products, in particular nonylphenol. U.S. Pat. No. 5,116,534 does not provide any hint whether or not said stabilizers are suited to increase the colour stability of the polymers.
WO-A-2009/138342 discloses the use of a combination of    a) a sterically hindered phenol bearing at least one sulfide group of the following formula
                wherein        R1 is C8-C12 alkyl        R2 is hydrogen, C1-C12 alkyl, cyclohexyl, 1-methylcycloheyl, benzyl, α-methylbenzyl, α,α-dimethylbenzyl or —CH2—S—R1         R3 is C1-C12 alkyl, benzyl, α-methylbenzyl, α,α-dimethylbenzyl or —CH2—S—R1, and        R4 is hydrogen or methyl, and            b) a styrenated diphenylamine of the formula
for stabilizing emulsion polymers or rubber latices. However, diphenyl amines are toxicologically critical.
JP 2010/077334A also discloses combinations of different stabilizers comprising a sulfur containing phenol-based antioxidant and an amin-based anti-ageing agent. Allegedly the use thereof for stabilizing nitrile rubbers results in an improved Mooney stability and an improved colour stability. With regard to the amounts of the sterically hindered phenols and amin-like stabilizers to be used a broad range is claimed. There is no disclosure or teaching provided how to generate further synergistic effects by using combinations of specific antioxidants.
EP-A-0 439 427 describes aqueous emulsions comprising 10 to 40% by weight, based on the emulsion, of antioxidants which comprise at least (A) one phenolic antioxidant, and/or (B) one thio dipropopionic acid ester and/or (C) an organic phosphite, besides 0.25 to 10% by weight of a surfactant being a salt of an organic acid, and 0.25 to 10% by weight of an alcohol. It is emphasized that such aqueous emulsion are storage stable, may be produced easily and are well suited for stabilizing a broad variety of polymers
WO 2005/023886 A focuses on the stabilization of a) methylmethacrylate-butadiene polymers or styrene graft polymers using b) a sterically hindered phenolic antioxidant of formula (I), (II) or (III) or a mixture thereof and c) a thioether differing from that of formula (II).
with n being from 1 to 10

In WO-A-2002/14419 salts of sterically hindered phenols are used for stabilizing rubbers. The stabilizers are characterized by comprising at least two phenolic hydroxy groups. Preferred sterically hindered phenolic compounds are those of the following formula
wherein R1, R2, and R3 may identical or different, R1, R2 are C1-C12 alkyl or C5-C8 cycloalkyl and R3 is hydrogen, C1-C8 alkyl or C5-C6 cycloalkyl. However, there is neither any disclosure nor any teaching how to further improve the stabilizing efficiency by using specific stabilizers in combination.
WO-A-2001/081458 discloses liquid stabilizing mixtures for organic polymers comprising    a) a liquid compound belonging to the group of sterically hindered phenols consisting of esters or mixtures of esters having general formula (I)
                wherein        R1 and R2 are the same or different, represent a linear or branched C1-C18 alkyl group;        R3 represents a linear or branched C8-C18 alkyl group, or one of the following groups:        
                wherein m and n are an integer ranging from 0 to 11, extremes included, and m+n is 10 or 11, and p is 12 or 13;            b) a solid compound belonging to the group of sterically hindered phenols having the following formula
                wherein n is an integer ranging from 0 to 10, extremes included.        
The stabilising mixture is said to be liquid and it is prepared by heating the components together. An inherent problem associated with this mixture is an insufficient compatibility with aqueous polymer dispersions. The use of such liquid preparation is inherently inefficient in the stabilisation of aqueous polymer dispersions as the antioxidant mixture is not fully compatible with an aqueous polymer dispersion. It is further on disclosed in WO-A-2001/81458 that such mixture may be used in combination with further stabilizers, and 19 different classes of compounds with more than hundred different antioxidants are listed as potential further stabilizers. There is no disclosure whether and if yes which specific combination of stabilizers might be suited to improve the stabilizing effect on rubbers synergistically.
In summary, it can be said that neither a process nor any stabilizing system being toxicologically unproblematic have been described up to now which allow to provide an improved stability to rubbers with regard to Mooney viscosity stability and at the same time colour stability.
It was therefore the object of the present invention to provide unsaturated rubbers having a good storage stability with regard to Mooney viscosity and colour stability, which do not encompass toxicologically and environmentally hazardous compounds and at the same time dispose of unchanged good processing properties, i.e. a good vulcanization profile and advantageous mechanical properties.